Occupational Risk Factors for Skin Cancer: A Comprehensive Review

Abstract

Public health and clinical medicine should identify and characterize modifiable risk factors for skin cancer in order to facilitate primary prevention. In existing literature, the impact of occupational exposure on skin cancer, including malignant melanoma and non-melanoma skin cancers, has been extensively studied. This review summarizes the available epidemiological evidence on the significance of occupational risk factors and occupations associated with a higher risk in skin cancer. The results of this review suggest that there is sufficient epidemiological evidence to support the relationship between the increased risk of non-melanoma skin cancers and occupational exposure to solar radiation, ultraviolet radiation, ionizing radiation, arsenic and its compounds, and mineral oils. Occupational exposure to pesticides and polychlorinated biphenyls appears to provide sufficient epidemiological evidence for melanoma, and a higher risk of melanoma has been reported among workers in petroleum refining and firefighters. This comprehensive analysis will establish a foundation for subsequent investigations and developing targeted interventions of focused preventive measures against skin cancer among the working population.

INTRODUCTION

Skin cancer is characterized by the abnormal growth of skin cells.1 There are different types of skin cancers, such as basal cell carcinoma (BCC), squamous cell carcinoma (SCC), and malignant melanoma (MM), each originating from various skin cell types and each presenting different characteristics. Although many skin cancers are treatable, they can cause substantial morbidity without early detection, particularly if advanced.

The risk factors for skin cancer include overexposure to ultraviolet (UV) radiation from the sun, artificial sources such as tanning beds, fair skin tone, severe sunburns, living closer to the equator or in high-altitude areas where UV radiation is more intense, a large number of moles or atypical moles (dysplastic naevi), family history, and conditions or medications that suppress the immune system.2 In addition to well-established risk factors, job-related factors are considered modifiable risk factors for skin cancer. The concentration of carcinogens in workplaces is typically higher than that in non-work settings. Occupational cancer can develop when workers are exposed to specific risk factors or a combination of these factors. Fortunately, almost all occupational exposure to carcinogens is theoretically preventable through appropriate protective measures. However, this is only possible when occupational risk factors are properly recognized.3

Therefore, this review aimed to summarize the available epidemiological evidence on occupational risk factors and high-risk occupation for skin cancer (Fig. 1). In addition, we reviewed the evidence for occupations associated with a higher risk for skin cancer. Given that mechanisms of skin cancer development might differ depending on the types of skin cancers, as well as exposure, review was conducted separately to MM and non-melanoma skin cancer (NMSC). This distinction could help in understanding carcinogenesis pathways and developing more targeted and effective prevention strategies, addressing specific occupational risks, and implementing effective interventions for each skin cancer category.

Fig. 1. Schematic diagram summarizing the possible occupational risk factors for skin cancer.

NMSC = non-melanoma skin cancer, MM = malignant melanoma.

EVIDENCES IN PREVIOUS RESEARCH

Solar and UV radiation

Solar rays span a spectrum of electromagnetic waves with infrared (50%), visible (45%), and UV radiation (5%). UV radiation can be divided into three types: long-wave UVA (315–400 nm), medium-wave UVB (280–315 nm), and short-wave UVC (100–280 nm). The Earth’s ozone layer absorbs UVB and UVC below 290 nm, controlling the amount that reaches the surface. This absorption regulates the amount of UVC and UVB radiation reaching the Earth’s surface. However, observations indicate ongoing depletion of the ozone layer due to the accumulation of carbon dioxide in the atmosphere, resulting in increased levels of UVB reaching the Earth’s surface.4

UV radiation can cause considerable damage to the skin and long-term cumulative exposure can directly affect the development of skin cancer. Short-term intense sun exposure is an important cause of melanoma and BCC, whereas long-term continuous sun exposure is associated with development of SCC.5 The International Agency for Research on Cancer (IARC) classifies UV radiations as Group 1 carcinogens.6 Typical skin cancers induced by UV radiation include NMSC such as BCC and SCC.

NMSC

In previous IARC papers, the causal relationship of UV radiation with BCC, SCC, and actinic keratosis (AK) was assessed through numerous epidemiological studies.6 These studies have demonstrated that UV radiation is a strong risk factor for NMSC, particularly in individuals with lighter skin tones. A review conducted by the World Health Organization (WHO) and the International Labor Organization (ILO) postulated a causal relationship between work-related UV exposure and NMSC.7 Recognition of occupational exposure to UVR primarily pertains to outdoor workers, such as agricultural workers, construction workers, and others who spend significant amounts of time outdoors during peak sunlight hours. These individuals are at a higher risk of prolonged and intense UV exposure compared to the general population. However, most epidemiological studies and evaluations are based predominantly on descriptive data from Caucasian populations, indicating the lack of research among non-Caucasian ethnicities due to low incidence.8

According to a systematic review by Loney et al.9 involving 19 studies, 18 reported an increased risk estimate for BCC and/or SCC among outdoor workers. All four studies reporting risk estimates for specific occupations indicated a significantly higher risk among agricultural workers. According to a case-control study comparing the prevalence of NMSC between outdoor and indoor workers in Europe, mountain guides (odds ratio [OR], 5.9; 95% confidence interval [CI], 2.4–14.6), gardeners (OR, 4.0; 95% CI, 1.7–9.5), and farmers (OR, 2.3; 95% CI, 1.0–5.0) exhibited significantly higher risks of NMSC compared to indoor workers.10 In another case-control study targeting Europeans, outdoor worker groups, excluding agricultural and construction workers, exhibited significant increases in AK (OR, 1.55; 95% CI, 1.09–2.18) and BCC (OR, 1.53; 95% CI, 1.39–2.41), whereas agricultural and construction workers showed higher risks for AK (OR, 2.58; 95% CI, 1.93–3.44), SCC (OR, 2.77; 95% CI, 1.97–3.88), and BCC (OR, 1.83; 95% CI, 1.80–2.96). Moreover, a significant elevation in all skin cancer risks was observed with over 5 years of exposure to outdoor work (AK, 3.45; BCC, 3.32; SCC, 3.67; melanoma in situ, 3.02; MM, 1.97; all P < 0.05).11 Likewise, analysis of cancer registry data from Germany showed that relative risks (RRs) of NMSC incidence among outdoor workers were significantly higher for males with BCC (RR, 2.9; 95% CI, 2.2–3.9) and SCC (RR, 2.5; 95% CI, 1.4–4.7) and for females with BCC (RR, 2.7; 95% CI, 1.8–4.1) and SCC (RR, 3.6; 95% CI, 1.6–8.1), all showing statistically significant elevations.12

MM

There is convincing evidence that UV exposure is a major environmental factor in the development of MM, especially in high-risk populations, although not all melanomas are directly related to sun exposure.13

However, according to a systematic review and meta-analysis from the WHO/ILO Joint Estimates of the Work-related Burden of Disease and Injury, reported research on “occupational” exposure to solar UV radiation commonly shows weak, null, or negative associations with melanoma incidence, leading to an evaluation of “limited evidence for harmfulness” and a current low quality of available evidence.7 Subsequently, a case-control study from Italy published in 2023 reported no association between occupational UV exposure and melanoma.14

Summary of evidences

Exposure to sunlight and UV radiation has been extensively documented as the primary cause of skin cancer. Both BCC and SCC were strongly associated with occupational UV exposure. However, the relationship between occupational exposure to solar UV radiation and MM is unclear, but intense UV exposure can increase the risk; therefore, caution must not be neglected.

Ionizing radiation

Exposure to ionizing radiation causes various types of cancer, including leukemia, breast cancer, thyroid cancer, and lung cancer.15 The danger of ionizing radiation in skin cancer has been demonstrated in cases caused by radiation therapy of the head and neck,16 X-ray treatment of ringworms of the scalp,17 and atomic bombs.18

NMSC

Skin cancer caused by ionizing radiation usually manifests as NMSC, especially BCC, rather than melanoma.19 The latent period of BCC can range from 2 to 70 years after exposure to radiation of up to 30 Gy.20 The incidence also increases with co-exposure to sunlight, sun-sensitive skin, radiation exposure before the age of 20 years, and genetic factors.19 In addition, SCC is reportedly associated with radiation exposure. The latent period is estimated to be approximately 20 years, and radiodermatitis often presented beforehand.16

A cohort study of workers in Russian nuclear production facility who are occupationally exposed to low-dose ionizing radiation showed that the incidence of NMSC increased by more than twice if exposed to ionizing radiation at cumulative doses above 2.0 Sv.21 More recent results of analysis with workers in same facility chronically exposed to low dose ionizing radiation occupationally, showed that the excess RR of BCC per unit skin absorbed dose of external exposure (ERR/Gy) was 0.57 (95% CI, 0.24–1.06), whereas no significant association was shown for SCC.22

Because medical practitioners are frequently exposed to radiation, research has been conducted to investigate the risk of skin cancer in these occupations. A 2005 cohort study suggested that radiologic technologists had an elevated risk of BCC if they first worked during the 1950s or earlier, indicating that they were exposed to a higher dose of ionizing radiation at that time.23 The association was stronger among individuals with lighter compared to darker eye and hair colour. Another study limited the participants to Caucasian technologists in the U.S., and the results were consistent with those of other studies.24 The risk of BCC increased with radiation exposure before the age of 30 years and prior to 1960.

MM

Although MM is rarely associated with exposure to ionizing radiation, melanoma has been reported after radiation therapy.20 Fink and Bates25 suggested a causal relationship between ionizing radiation and melanoma by estimating radiation exposure with a relative risk of leukemia. An elevated relative risk of leukemia is positively correlated with an elevated relative risk of melanoma.

Occupational studies have been conducted on this topic. The relative risk of melanoma was 1.8-fold for radiologic technologists who started working before 1950, when the estimated occupational radiation exposure was higher. The relative risk was 2.4 for those who had worked more than 5 years before 1950, and 1.4 if they did not wear protective shields such as lead aprons.26 Moreover, pilots and flight attendants are known to have a higher prevalence of melanoma, considering their exposure to cosmic radiation,27 and a meta-analysis estimated a standardised incidence ratio over 2.28 However, in studies on flight-based occupations, it is difficult to distinguish the effects of ultraviolet radiation.

Summary of evidences

Overall, ionizing radiation is widely recognized as a cause of NMSC, especially BCC. Studies on radiology technologists and other medical professionals support the effects of radiation. The risk of MM could also be associated with exposure to ionizing radiation; however, the evidence for this is insufficient.

Arsenic and its compounds

While the general population ingests arsenic through food and beverages averaging between 20–300 mcg/day, occupational exposure to arsenic primarily occurs through inhalation and dermal absorption.29,30 The carcinogenic mechanism of arsenic involves the induction of oxidative stress, immune dysfunction, chromosomal abnormalities, and altered growth factors.31 Arsenic and inorganic arsenic compounds have been classified as group 1 carcinogens by the IARC, with sufficient evidence for NMSC.32

NMSC

The most prevalent skin cancers induced by arsenic include, SCC, BCC, and Bowen disease (SCC in situ).33 Smith et al.34 investigated the impact of arsenic exposure in drinking water on cancer mortality in Region II of Chile, revealing increased mortality rates for skin cancer (standardized mortality ratio [SMR], 7.7; 95% CI, 4.7–11.9 for men; SMR, 3.2; 95% CI, 1.3–6.6 for women). Furthermore, Kim et al.35 studied 124 biopsy-proven NMSC cases and 125 matched controls and discovered higher levels of certain arsenic forms, such as total inorganic arsenic, trivalent and pentavalent arsenic, and monomethylarsonic acid, in the NMSC group. However, all of these results are based on non-occupational exposure through ingestion rather than occupational exposure.

While the study conducted with 618 NMSC cases and 527 hospital-based controls showed no significant association between occupational airborne arsenic exposure and NMSC (OR, 1.94; 95% CI, 0.76–4.95), the adjusted OR significantly increased by co-exposure to sunlight in workplace (OR, 10.22; 95% CI, 2.48–42.07).36 This dramatic increase in risk can be attributed to the synergistic effect of arsenic and UV radiation. Additionally, arsenic can act as a co-carcinogen, enhancing the carcinogenic effects of UV radiation. This combined exposure leads to a higher likelihood of genetic mutations and the development of skin cancer. Karagas et al.37 conducted a case-control study in New Hampshire to investigate the impact of environmental arsenic levels on NMSC (284 SCC, 587 BCC, and 524 control cases). While there was no statistically signification association in individuals with toenail arsenic concentrations above the 97th percentile (OR, 2.07; 95% CI, 0.92–4.66 for SCC; OR, 1.44; 95% CI, 0.74–2.81 for BCC), this may be linked to challenges in accurately modelling the dose-response relationship due to limited subjects at extreme exposure levels.

MM

Only a few studies have investigated the association between melanoma and arsenic exposure, and have shown statistically insignificant associations, although some data suggested a possible association between melanoma and arsenical pesticides.38 Bedaiwi et al.39 collected cross-sectional data from 36,418 participants (no cancer, n = 35,572; MM, n = 244; NMSC, n = 602). The results of analysis indicated that individuals with higher urinary arsenic had an adjusted OR of 1.87 (95% CI, 0.58–6.05) for MM and 2.23 (95% CI, 1.12–4.45) for NMSC.39 Langston et al.40 mirrored similar findings, suggesting an insignificant association between arsenic and drinking water or environmental soil.

Summary of evidences

While extensive research has reported an association between arsenic exposure in drinking water and NMSC, evidence of relationship between occupational exposure to arsenic and skin cancer has been insufficient.

Pesticides

The widespread use of pesticides causes environmental pollution and poses health risks to human.41 Pesticides are delivered to the body via several routes. In particular, the skin is the most exposed body organ when spraying pesticides in the field. Therefore, there are large amount of reports that the incidence of skin cancer significantly increased among farmers who used pesticides.42 According to the IARC classification, occupational exposure to arsenic and polychlorinated biphenyls (PCBs) is a definite carcinogen for skin cancer,43 and these substances commonly comprise pesticides.

NMSC

Few studies have investigated the relationship between pesticides and NMSC. According to a study comparing British pesticide users with the general population, a significantly higher standardized incidence ratio (SIR) of NMSC was observed for both males (SIR, 1.11; 95% CI, 1.00–1.23) and females (SIR, 1.73; 95% CI, 1.06–2.82).44 However, the SMR was not statistically significant.

MM

A meta-analysis conducted in 2019 reported a significant association between pesticide use and cutaneous melanoma (CM) by analysing two case-control studies and seven cohorts.45 The risks were divided into four categories: herbicide-ever exposure, insecticide-ever exposure, any pesticide-ever exposure, and any pesticide-high exposure. The summary RRs for the categories are 1.85 (95% CI, 1.01–3.36], 1.57 (95% CI, 0.58–4.25), 1.31 (95% CI, 0.85–2.04), and 2.17 (95% CI, 0.45–10.36), respectively. Therefore, it has been suggested that individuals high exposed to pesticides have an increased risk of developing CM. In addition, according to a systematic review in 2020 on the correlation between pesticide applicators and cancer,46 the ORs of melanoma were 2.4 (95% CI, 1.2–4.9, trend P = 0.006) for mancozeb (≥ 63 days of exposure) and 2.4 (95% CI, 1.3–4.4, trend P = 0.003) for parathion (≥ 56 exposure days). For carbaryl, RR of melanoma was significantly higher with lifetime exposure exceeding 175 days (RR, 4.11; 95% CI, 1.33–12.75), ≥ 10 years (RR, 3.19; 95% CI, 1.28–7.92), or 10 days per year (RR, 5.5; 95% CI, 2.19–13.84), compared to individuals who had never used it. Another systematic review conducted in 2022 reported that men aged 60 years and older who were occupationally exposed to pesticides had an increased risk of death from melanoma compared to the general population (SMR, 2.1; 95% CI, 1.2–3.5).47 Additionally, according to another meta-analysis conducted in 2023, two studies from the International Agricultural Cohort Consortium (AGRICOH) were reviewed and indicated that women had a significantly higher incidence of melanoma compared to the general population (meta-SIR, 1.18; 95% CI, 1.01–1.38).48 The analysis only covered occupational exposure, and the type of pesticide was unspecified.

Pesticide use is high among farmers working outdoors, where co-exposure to solar radiation is common. This may further increase the risk of melanoma development. Case-control studies from Italy and Brazil found that people who used herbicides (glyphosate) and fungicides (mancozeb, maneb) had a higher risk of developing melanoma compared to controls (OR, 2.58; 95% CI, 1.18–5.65).49 Particularly, the risk was higher when sun exposure was combined with pesticide use (OR, 4.68; 95% CI, 1.29–17.0).

Summary of evidences

Numerous studies have been conducted over several decades and a correlation between pesticides and skin cancers has been established. The association with MM has been frequently reported. As the same trend was observed in the results after controlling for variables related to solar radiation exposure, the influence of pesticides can be explained. However, most information on exposure is obtained through questionnaires, making it difficult to obtain strong trust. Moreover, as farmers spend a lot of time working outdoors in the sunlight, this factor cannot be completely excluded when evaluating the relationship between pesticides and skin cancer.38

Organic solvents

Organic solvents are carbon-containing substances that dissolve or disperse one or more substances.50 Trichloroethylene (TCE), tetrachloroethylene, carbon tetrachloride, methylene chloride, benzene, styrene, and similar compounds are examples of solvent constituents. They are commonly used in paints, varnishes, lacquers, adhesives, glues, degreasing agents, and cleaning agents to produce dyes, polymers, plastics, textiles, printing inks, agricultural products, and pharmaceuticals. The skin is a significant exposure route for organic solvents.

Some organic solvents are associated with cancers of the liver, kidney, lungs, haematopoietic system, and bladder.51 PCBs, which can also be found in certain industrial applications and pesticide formulations, are involved in various aspects of tumour promotion and progression and are classified as Group 1 carcinogens by the IARC.52 Particularly, “dioxin-like PCBs” activate aryl hydrocarbon receptors, which can modulate melanogenesis. This suggests a potential association between PCB exposure and an elevated risk of melanoma.

NMSC

Based on a consensus study reported by the Institute of Medicine, an association between TCE and NMSC was suspected, although only one study reported a statistically significant result.53 For the other organic solvents, six studies reported statistically insignificant associations.

MM

According to IARC Monograph volume 107, an association between PCB exposure and melanoma has been reported in several studies.52 However, recent studies reported contradictory results. In a meta-analysis published in 2017, even though the pooled SMR was statistically significant (SMR, 1.32; 95% CI, 1.05–1.64), the cohort studies used had several limitations in research design such as major confounders like sun exposure.54 Another meta-analysis in 2018 reported insignificant correlation: the summary RR in random effect model was 1.13 (95% CI, 0.91–1.35) and the results from occupational cohorts and general population studies are close, even though the occupational exposures include higher concentrations.55

For substances other than PCB, the evidence of their association with melanoma is limited. In a consensus report by the Institute of Medicine, a compilation of epidemiological studies was presented.53 Among the five studies on TCE exposure, statistically significant associations were reported in one case-control study, although the number of exposed cases was limited.56 Statistically insignificant associations were observed in two studies using tetrachloroethylene and dry-cleaning solvents. In eight studies on other solvents, there was a statistically insignificant overall increase in risk. Additionally, for toluene and xylene, there were reverse significant correlations, even though the number of cases was small (5 cases for Toluene, estimated RR, 0.4; 95% CI, 0.1–0.9; 3 cases for Xylene, estimated RR, 0.3; 95% CI, 0.1–0.8).53 These inverse associations may indicate a potential confounding factor reducing the observed risk. The mechanisms behind these observations could involve competitive inhibition of metabolic pathways essential for carcinogenesis or differences in exposure levels and bioavailability that reduce the effective dose reaching target tissues.

However, other studies have reported a significant association. For example, a case-control study on chlorinated solvent exposure investigated 3,730 cancer cases and 533 control subjects, including 103 cases of melanoma. A significant association between melanoma and TCE was observed (for any exposure, OR, 3.0; 95% CI, 1.2–7.2; for substantial exposure, OR, 3.2; 95% CI, 1.0–9.9).57 In a meta-analysis of melanoma risk in industrial worker, three studies analysing the chemical industry showed a SIR of 2.08 (95% CI, 0.47–9.24), similarly, the meta-analysis of eight studies in the same context reported a SMR of 2.01 (95% CI, 1.09–3.72).58 The identified hazardous factors were primarily associated with polyvinyl chloride and vinyl chloride monomers which are commonly used as intermediates or solvents in the synthesis of organic compounds in industrial processes.

Summary of evidences

Most organic solvents act as skin irritants, and many have been confirmed to be carcinogenic. However, evidence of their association with skin cancer is currently inconsistent and contradictory. Although PCBs were classified as Group 1 by the IARC in 2016, subsequent research showed controversial results regarding the association between occupational PCB exposure and melanoma. Therefore, additional studies are needed to strengthen and re-evaluate this evidence.

Mineral oils

Mineral oils (sometimes referred to as base oils, mineral base oils, or lubricant base oils) are chemically complex and have various combinations of straight and branched-chain paraffinic, naphthenic, and aromatic hydrocarbons, which are refined from petroleum crude oils.59 They are used in the manufacture of a wide range of products, such as lubricants and non-lubricants, for various applications. Engine, transmission, gear, hydraulic, and metalworking fluids are examples of lubricant products, whereas printing inks, tire oils, and agricultural spray oils are examples of non-lubricant products.

Mineral oils may contain polycyclic aromatic hydrocarbons (PAHs) and other chemical additives together with biocides to limit microbiological contamination.60 The carcinogenicity of oil-based fluids is determined by how basic oils are treated, and the compounds added to them to improve their performance as coolants and lubricants. The removal of PAH from mineral oils began in the 1950s when the association between machining fluids and skin cancer began to appear. Oils that have undergone solvent extraction and hydrotreatment have fewer PAH, which lowers their carcinogenic potential.

NMSC

In 1984, the IARC classified untreated and mildly treated oils as Group 1 carcinogens with sufficient evidence from human studies.61 A few years later, The National Institute of Occupational Safety and Health (NIOSH) came to a similar conclusion.62 The evaluation by the IARC and NIOSH was mostly based on evidence of SCC of the skin, especially scrotal cancer. However, a risk assessment for cancer results from the United Autoworker-General Motors (UAW-GM) cohort of 38,549 automobile manufacturing workers with potential exposure to metalworking fluids, reported elevations in skin cancer mortality from long-term occupational exposure to metalworking fluids.63

MM

Evidence of occupational mineral oil exposure and melanoma has been reported relatively recently, mainly from retrospective cohort studies targeting workers in the US aviation or automobile industries. In a retrospective cohort study of workers employed at a California aerospace company between 1950 and 1993, high levels of exposure to mineral oils increased the incidence of melanoma (rate ratio, 3.32; 95% CI, 1.20–9.24).64 Similarly, in a cohort of autoworkers followed from 1985 through 2004 for cancer incidence,60 the hazard ratio for the highest category of straight fluid was 1.99 (95% CI, 1.00–3.96), based on 14,139 white male participants with 76 incident cases of MM. These findings indicate that oil-based fluids, especially straight mineral oils, are associated with a higher risk of MM, based on quantitative measurements of the metalworking fluid.

Summary of evidences

Mineral oils, including metalworking fluids, contain PAHs and are prone to carcinogenicity when left untreated or mildly treated. Sufficient evidence has been reported in humans for SCC of the skin, especially scrotal cancer, since the mid-1900s. In contrast, there have been few studies on the risk of melanoma, but several recent retrospective cohort studies have reported an increased risk of melanoma in workers exposed to mineral oil.

Petroleum refining

The petroleum industry is significant in many countries and contributes to the production of various chemicals. However, petroleum refining workers face potential health hazards, including the risk of developing skin cancer. Occupational exposure in petroleum refining has been classified as Group 1 by the IARC, with limited evidence for MM and NMSC.65,66

NMSC

Regarding the relationship between petroleum refining and NMSC, few studies have primarily consisted of case reports and series. Davis67 reported the case of a refinery worker who was diagnosed with SCC after removing paraffin wax for over 20 years. Subsequent investigations at the plant revealed that many workers suffered from wax boils that developed into pigmented spots, wart-like growth, and epithelioma. Lione and Denholm68 reviewed ten cases of scrotal cancer in wax pressmen in petroleum refineries. All patients were pathologically confirmed to have SCC. The workers had a minimum exposure period of 15 years to crude wax containing 20–40% petroleum oil distillate, with the longest exposure period lasting 38 years.

MM

Several cohort studies have documented the relationship between petroleum refinement and risk of melanomas among Australian, U.S., and UK petroleum industry workers.69,70,71 In a systematic review, Schnatter et al.72 discovered that 10 of 17 studies indicated a higher incidence of melanoma among petroleum refinery workers (meta-RR, 1.23; 95% CI, 1.09–1.38), but the mortality rates were not statistically significant. A more recent systematic review also found an increased risk of melanoma (summary effect size, 1.90; 95% CI, 1.34–2.70) associated with working in the petroleum industry, whereas no such elevation was found within the subset related to refinery work.73 However, a meta-analysis of melanoma risk in industrial workers revealed increased mortality (summary SMR, 1.02; 95% CI, 0.81–1.28) and incidence (summary SIR, 1.23; 95% CI, 1.11–1.36) in the oil/petroleum industry, implying amplified risks considering the healthy worker effect.58

The increased risk of melanoma in the petroleum industry highlights the need to identify specific carcinogens that workers are primarily exposed to via vapour inhalation and skin contact. Mehlman et al.74 reviewed the causal relationship between exposure to chemicals in oil refining and MM, and identified exposure to chemicals such as benzene, aromatic hydrocarbons, PCBs, and heavy oils as contributing factors to the development of melanoma in oil-refining workers. PAH, classified as carcinogens by the IARC, disrupt cell membrane function and have carcinogenic, mutagenic, and immunosuppressive effects. PCBs also show sufficient evidence of their carcinogenicity in human melanoma, as mentioned above.

Summary of evidences

While some studies have indicated statistically significant relationship between petroleum refining and NMSC, the evidence is mostly based on old ones. However, there is sufficient evidence for a relationship between petroleum refining and melanoma, and chemicals such as PAH and PCBs appear to contribute to the development of melanoma in oil-refining workers.

Firefighters

Firefighters are exposed to many occupational hazards, and are already categorized as carcinogenic to humans (Group 1) in volume 132 of the IARC Monographs (mesothelioma, bladder cancer).75 There is a positive association between firefighters and several cancers, including MM, as well as substances such as PAH and benzene to which firefighters are exposed to.

NMSC

In the case of NMSC, the results related to the risk of incidence were not consistent, and no studies with increased mortality were identified. In a meta-analysis in 2020, when the meta-RR of skin cancers combined was stratified into regions, statistically significant results were found in Australia, New Zealand, and Korea (RR, 1.43; 95% CI, 1.27–1.58).76 A Swedish cohort study also reported a statistically significant increase in incidences of NMSC amongst firefighters (SIR, 1.48; 95% CI, 1.20–1.80).77 When stratified by employment period, the risk of NMSC was significantly elevated in those with a working period of 10–19 years (SIR, 1.82; 95% CI, 1.21–2.62) and 20–29 years (SIR, 1.56; 95% CI, 1.09–2.17). However, the risk was insignificantly elevated among firefighters who worked for 30 years or more.

MM

According to meta-analyses published in recent years on firefighters, the incidence of melanoma has generally increased,78,79 but the mortality rate has not.76,78,79 However, in a Swedish cohort study, the risk of melanoma among firefighters was statistically insignificant, and there was no association between melanoma incidence and duration of employment.77

Notably, the overall incidence of melanoma has decreased in firefighters over time.77,80,81 For example, a decrease in the incidence of malignant skin melanoma was observed among Swedish firefighters was observed.77 This could be related to incomplete Swedish cancer registration and explained by the healthy worker effect, which can be attributed to a lower risk if individuals are over 70 years old or have worked for more than 30 years.82 In addition, the recent trend of increasing periodic health checkups and improvements in protective measures to prevent exposure to carcinogenic substances can affect the incidence of skin cancer in firefighters.79

Summary of evidences

Firefighters showed a higher incidence of MM than the control group. This may be the case due to the exposure of firefighters to several carcinogens occupationally, such as PCBs and PAHs; however, many other factors are also involved. Current evidence regarding NMSC remains unclear and controversial. To obtain a clear conclusion regarding the occupational skin cancer risk in firefighters, it is necessary to comprehensively consider the healthy worker effect, carcinogen exposure, and regional differences.

CONCLUSION

A substantial degree of research has been conducted on the influence of occupational factors on skin cancer. The results of this review suggest that there is sufficient epidemiological evidence to support the relationship between the increased risk of NMSC and occupational exposure to solar radiation, ultraviolet radiation, ionizing radiation, and mineral oils. Occupational exposure to pesticides and PCBs appears to provide sufficient epidemiological evidence for melanoma, and a higher risk of melanoma has been reported among workers in petroleum refining and firefighters. However, the association between skin cancer and occupational exposure to other organic solvents and arsenic remains unclear. In addition, other occupational factors have been documented to be related to skin cancer, including occupational exposure to shale oil83 and repetitive mechanical stress.84 However, these factors were not covered in this review due to the limited availability of fragmented evidence.

Future research on occupational risk factors should address the following questions to build upon the robust evidence. The incidence of skin cancer is high; however, except for MM, the mortality risk is low. Therefore, mortality is a poor proxy for the risk of treatable cancers and is not an optimal outcome measure in etiological studies.63 Moreover, in relation to high-risk occupations, the healthy worker survivor effect seems to be strong; therefore, it should be considered in the research design, analysis, and interpretation of results. Occupational risk factors often include simultaneous exposure to solar UV radiation, which is known to be the strongest risk factor, and it can be difficult to distinguish their effects. Some studies have addressed simultaneous exposure to UV radiation;19,36,38,47,54,85 however, a more mechanistic understanding of this is warranted.